1. Field of the Invention
The present invention relates to improving the properties, in particular the crazing resistance, of transparent, acrylic, monolithic or laminated sheet materials, based on a methyl methacrylate homopolymer or copolymer, in particular polymethyl methacrylate homopolymer (PMMA). It also relates to these materials having improved properties, to glazing, in particular aircraft glazing based on these materials, to a process for treating a conventional material of this type so as to improve the properties thereof and, in general, to a process for improving the properties of these materials.
2. Description of the Background
In the description which follows, we will usually refer to the outer panels of airplane windows, which represent the preferred application of the present invention, but it goes without saying that it is not limited to aircraft glazing (windows, windshields, helicopter canopies, cockpit sides) and that it encompasses all other possible applications of these sheet materials based on PMMA or the like: glazing of all types, canopies, etc.
At the present time windows are manufactured from drawn PMMA. The drawing strengthens the PMMA material, in particular against notch propagation, and increases the crazing resistance. The use of this material is justified by a good mechanical strength/weight ratio and relatively easy processing: peripheral machining of monolithic sheets and mounting them as double glazing in a silicone seal.
However, the surface resistance of this drawn acrylic material remains relatively poor: abrasion, scratching, crazing and appearance of cracks degrade the optical quality of the window, as the outer skin, and in certain types of lighting prevents vision. At this stage, the windows must be removed—an expensive operation—in order either to be repaired by machining or to be replaced. The current situation has two consequences for airlines: lack of passenger comfort and negative brand image when a window is highly degraded; extra cost to repair and/or replace it.
Since 1980, there have been several crazing crises affecting most large airlines and resulting in windows having to be replaced after 2000 hours of flying compared with 10 000 to 15 000 hours of flying in normal weather. Hypotheses have been put forward to explain these crises:
The main origin of crazing (a major phenomenon with pressurized aircraft) is due to a water absorption/desorption phenomenon on the outer surface of the window, this phenomenon being magnified by the following cycles:
atmospheric pressure+high ground humidity: water absorption;
low pressure+low humidity at 10 000 m: water desorption.
This water ingress/egress phenomenon has by itself little effect on the life of the window, as it leads to an equilibrium in the water concentration. However, the water has an affinity for the constituent polymer of the window, (acting as a “plasticizer”) and will “dissolve” in the polymer and then diffuse therein. However, during this absorption/desorption phenomenon and this diffusion, the water entrains with it sulfuric acid—a powerful oxidizing agent—whose concentration in the air is very greatly increased in the case of volcanic eruptions, which are not so rare. Water desorption can then be accelerated by the sulfuric acid which acts as a dessicant and results in high stresses leading to crazing. The surface of the window is also subjected to fitting stresses. By therefore being subjected to deformation and volume variations, and thus stressed in the presence of sulfur derivatives, the surface of the window inevitably undergoes this crazing.
To overcome this disadvantage that the drawn PMMA material has, of thus leading to crazing, two types of solutions have been proposed hitherto:
(1) the selection of an acrylic raw material of very high quality, which is highly crosslinked, and therefore has a low water absorption, combined with very high surface quality obtained by polishing after stress-free diamond machining;
(2) protective varnishes which are abrasion-resistant and eliminate any risk of crazing, since they exhibit little or no exchange with water, these varnishes possibly being soft (polyurethane) or hard (polysiloxane); or else protective films which also form a physical barrier with respect to the window.
Solution (1) is not entirely satisfactory.
Nor, in the case of high sulfuric acid pollution levels, is solution (2) any more satisfactory because of the presence of a “thickness”, the surface optical quality being inferior to that of a bare window. In addition, the varnish is deposited using flow-coating or spraying techniques, which operations are made complex by the need to also protect the edge of the window in the recessing. Moreover, the varnishing must necessarily be carried out in a clean (class 1000) room since dust on the surface at the time of application will generate a major optical defect (tent peg, harl, etc.) which is particularly visible by the magnifying glass effect. The varnishes can be neither repaired nor regenerated, making it necessary when the surface has degraded to remachine it.
Thus, the application of a film or varnish involves an extra cost which it is desired to avoid.